This invention relates to liquid heat systems and in particular to those employing thermal storage at temperatures substantially above that of the heated system. It is well known that heating equipment utilizing thermal storage is highly desirable in that an infrequent or a periodic heat source can be utilized in addition to providing a means to defer or reduce peaks on energy supplied by utility systems.
Examples of a periodic heat which is made usable through a storage system include waste heat from incinerators, prime movers, and heat pump outputs.
Systems of this type are disclosed in the following U.S. Pat. Nos. 3,422,248, 3,630,275, 3,298,432, 3,411,571, 2,911,513, and 2,461,774. The above systems; in particular, the U.S. Pat. Nos. 3,298,431 and 3,411,571 employ thermal storage at substantially higher temperatures than the systems which are supplied with the stored thermal energy. Use of a high temperature storage is highly advantageous as it reduces the physical size of the storage unit, and provides a large temperature difference which improves heat transfer and response to demands for heat. Most heated systems providing a thermal load utilize liquids, such as water, which undergo phase changes through boiling at temperatures near those required to satisfy system needs. The resultant pressure increases due to vaporization produced by boiling of heated system liquids, in addition to being dangerous, result in unsteady flow of the liquid, substantial mechanical forces on the system and noise due to pressure pulsations in the heated system. Approaches disclosed in the above patents employ elaborate, expensive, and in many cases unreliable techniques in transferring heat.
The major technical problem involved in heating volatile liquids from high thermal potentials or temperatures is well known in the art as thermal "matching". In order to accomplish heat transfer without phase change it is necessary either to provide increased surface area or flow rates, or to employ other methods which effectively adjust the hydro-dynamic films between the liquid and heated surfaces to provide proper thermal gradiant for preventing the highly undesirable boiling. It is also necessary to accomplish this in a thermally efficient manner by varying the heat transfer conditions with load, since ordinary cooling of the films would result in substantial heat losses.
A practical solution to this problem has been provided by the systems disclosed in the above U.S. Pat. No.s. 3,422,248, and 3,630,275 patents in which the applicant was a co-inventor and are hereby incorporated by reference to this application. The schemes disclosed in these patents are "off-peak" stored energy heating systems which utilize a proprietary condensing system which essentially "matches" the energy contained in the higher temperature storage with that required in the heated water system. However, a characteristic of the system disclosed requires that an initial heat demand results in a certain amount, although controlled, of liquid flashing into vapor within the storage tank heat exchanger. An additional difficulty encountered in prior art systems involves pressure differential between thermal storage and heat utilizing systems. For example, in a water heating system employing heat storage at 280.degree. F. (137.5.degree. C.) in order to provide 160.degree. F. delivered hot water, heated or system water, the "system" pressure cannot be reduced below saturation values for water at its highest temperature at any point in the system. Due to variation in film coefficients in various parts of the heat exchange system, it is necessary to maintain a substantially higher pressure than that corresponding to saturation at the load side or delivered water temperature if flashing is to be avoided. However, in many applications, it is necessary to operate the heated water system at a relatively high static pressure. In these situations the methods for controlling "flashing" disclosed in U.S. Pat. Nos. 3,422,248, and 3,630,275, are unsatisfactory, due to the high costs incurred in manufacturing condensing equipment which will operate satisfactorily at the higher pressures. Heretofore, although high pressure heat exchangers were economically attractive and available, the flashing or thermal matching problem precluded their use. However, application of the novel concepts of the invention allows use of conventional heat exchangers over a wide range of liquid temperatures and pressures without flashing or phase change.
Generally speaking, therefore, the attendant difficulties resulting from utilizing high temperature storage to supply heat for medium or low temperature systems, at reasonable cost and providing necessary reliability is still a substantial problem in the industry. As indicated above, the present state of the art has produced only partial solutions to the problem.